1. Field of the Invention
The present invention relates to a variable-stroke crank mechanism for use in a radial-plunger variable-displacement fluid device having cylinders and plungers whose axes extend radially around a crankshaft, and more particularly to a variable-stroke crank mechanism suitable for use in a radial-plunger variable-displacement fluid device of the double crankpin type.
2. Description of the Prior Art
Radial-plunger variable-displacement fluid devices in which cylinders and plungers have axes extending radially around a crankshaft are well known in the art of hydraulic pumps, hydraulic motors, and compressors. For example, U.S. Pat. No. 5,076,057 discloses a plunger-type hydraulic unit as a radial-plunger variable-displacement fluid device.
In the disclosed plunger-type hydraulic unit, an off-center crankpin is mounted on a rotatable crankshaft, and an eccentric collar is mounted on the crankpin. Plungers are coupled to a connecting ring that is rotatably mounted on the eccentric collar, and slidably inserted in respective cylinders fastened to a casing.
When the crankshaft rotates, the crankpin and the eccentric collar also rotate in unison with the crankshaft. The plungers now reciprocate in the respective cylinders as the connecting ring revolves around the axis of the crankshaft. The plunger-type hydraulic unit includes a mechanism for rotating the eccentric collar in unison with the crankshaft.
Since the reciprocating stroke of the plungers can be varied by adjusting the rotational position of the eccentric collar on the crankpin, the above mechanism is used as a stroke adjusting mechanism for adjusting the rotational position of the eccentric collar.
In the conventional radial-plunger variable-displacement fluid device, the single crankpin is mounted on the crankshaft, and the plungers and cylinders are arrayed radially around the crankpin. The radial-plunger variable-displacement fluid device is often required to have a large displacement. To increase the displacement, however, it is necessary to increase the diameters of the plungers and cylinders or the reciprocating stroke of the plungers.
To increase the diameters of the plungers and cylinders, it is then necessary to prevent adjacent ones of the cylinders from interfering with each other. Accordingly, the plungers and cylinders have to extend radially outwardly for an increased displacement, resulting in an increase in the radial dimension and hence size of the radial-plunger variable-displacement fluid device.
To increase the reciprocating stroke of the plungers, it is necessary to increase the distance by which the crankpin is radially displaced off-center from the axis of the crankshaft. Increasing the off-center distance displaces the plungers and cylinders radially outwardly and increases the length of the cylinders radially outwardly, with the result that the radial-plunger variable-displacement fluid device becomes larger in size.
If two crankpins are mounted on a crankshaft and a radial array of plungers and cylinders is arranged radially of each of the crankpins, then it will be possible to increase the displacement of the device without increasing the size of the device.
However, since two stroke adjusting mechanisms are required in combination with the two respective arrays of plungers and cylinders for adjusting the rotational positions of the eccentric collars on the crankpins to adjust the reciprocating stroke of the plungers, the stroke adjusting mechanisms will be complex in structure and control.
The conventional stroke adjusting mechanism has a drive means extending over the crankshaft to an end thereof for adjusting the rotational position of the eccentric collar. If the same stroke adjusting mechanism design is used for adjusting the stroke of each array of plungers in a double-crankpin radial-plunger variable-displacement fluid device, then the two arrays of plungers and cylinders need to be associated with respective stroke adjusting mechanisms with the drive means extending to the opposite ends of the crankshaft. Therefore, the stroke adjusting mechanisms are rendered complex in structure, and their operation control is also complicated.